1. Field of the Invention
The present invention generally relates to a liquid crystal optical element and a three-dimensional display system including the liquid crystal optical element. More particularly, the present invention relates to a liquid crystal optical element with no polarizers and a three-dimensional display system including such a liquid crystal optical element.
2. Description of the Related Art
Liquid crystal displays (LCDs) are relatively thin display devices with comparatively low power dissipation. By taking advantage of these features, LCDs are currently used extensively in a broad variety of applications including office automation (OA) appliances such as word processors and personal computers, mobile telecommunications units such as personal digital assistants, and camcorders with a liquid crystal monitor.
Most of the LCDs currently used normally include a nematic liquid crystal layer. However, since a nematic liquid crystal material has a relatively low response speed, the conventional LCDs are inferior in moving picture display capability than CRTs.
This is why a smectic liquid crystal material with a high response speed has recently been adopted more and more often as an alternative material to the nematic liquid crystal material with a relatively low response speed. Among other things, smectic liquid crystal materials, exhibiting ferroelectricity or antiferroelectricity, show spontaneous polarization, and respond to an applied electric field due to interactions between the spontaneous polarization and the electric field. Thus, the smectic liquid crystal materials can respond as quickly as within about 1 ms.
Examples of optical elements using ferroelectric liquid crystal materials include a surface-stabilized ferroelectric liquid crystal (SSFLC) optical element. The SSFLC optical element is characterized by exhibiting bistability with respect to an applied electric field and switching between the two stabilized states as quickly as within about 1 ms.
However, the electrooptical response of the SSFLC optical elements exhibiting the bistability is limited to switching between bright and dark states. Accordingly, it is difficult for such optical elements to display grey-scale tones by controlling the applied voltages.
In contrast, an antiferroelectric liquid crystal material is characterized by exhibiting tristability, which is achieved by electric field induced phase transition between an antiferroelectric phase with no electric field applied and a ferroelectric phase with an electric field applied, and switching between two of the three stabilized states as quickly as the SSFLC optical elements (i.e., within about 1 ms).
The antiferroelectric liquid crystal material can contribute to driving an LCD by a simple matrix addressing technique when allowed to switch with a biased electric field applied. In addition, the antiferroelectric liquid crystal material can also contribute to displaying grey-scale tones by the simple matrix addressing technique if its state in which the antiferroelectric and ferroelectric phases coexist is controlled through applied voltages. For example, Y. Yamada et al., “A Full-Color Video-Rate Antiferroelectric LCD with Wide Viewing Angle”, SID '95 Digest, pp. 789-792, discloses a 6-inch liquid crystal optical element which can conduct a display operation in full colors with such an antiferroelectric liquid crystal material used in a simple matrix addressing mode.
A liquid crystal optical element that needs no polarizers by using such an antiferroelectric liquid crystal material was proposed. Generally speaking, polarizers are expensive optical members. Accordingly, when no polarizers are needed, the manufacturing cost of liquid crystal optical elements can be cut down. In addition, the decrease in yield, which would otherwise be caused by the foreign matter entering the LCD while the polarizers are being bonded, can be substantially eliminated.
For example, Japanese Laid-Open Publication No. 8-122830 discloses a liquid crystal optical element including two liquid crystal cells, in each of which an antiferroelectric smectic liquid crystal material with a dichroic dye is enclosed hermetically, and having no polarizers. FIGS. 8 and 9 schematically illustrate the liquid crystal optical element 500 disclosed in Japanese Laid-Open Publication No. 8-122830.
As shown in FIG. 8, the liquid crystal optical element 500 is obtained by stacking a liquid crystal cell 500a on another liquid crystal cell 500b. 
Each of these two liquid crystal cells 500a and 500b includes two substrates 510 and 520, an antiferroelectric smectic liquid crystal layer 530 provided between the substrates 510 and 520, and two electrodes 512 and 522 provided between the substrates 510, 520 and the liquid crystal layer 530. Although not shown in FIG. 8, an alignment film is provided on each of these electrodes 512 and 522.
As shown in FIG. 9, each of the two liquid crystal layers 530 includes liquid crystal molecules 531 and dichroic dye molecules 532, absorbs an incoming polarized light ray of which the plane of polarization is parallel to the absorption axis thereof, and transmits an incoming polarized light ray of which the plane of polarization is parallel to the transmission axis thereof. Also, the liquid crystal cells 500a and 500b are arranged such that a normal defined perpendicularly to the smectic layers 534 of the liquid crystal layer 530 in the liquid crystal cell 500a and a normal defined perpendicularly to those of the liquid crystal layer 530 in the other liquid crystal cell 500b cross each other at right angles.
While no voltage is being applied to the liquid crystal layers 530, the absorption axes of the liquid crystal layers 530 of the liquid crystal cells 500a and 500b cross each other at right angles, thus realizing a dark-state display.
On the other hand, when a predetermined voltage is applied to the liquid crystal layers 530, the absorption axes of the liquid crystal layers 530 of the liquid crystal cells 500a and 500b no longer cross each other at right angles, thus realizing a bright-state display.
Japanese Laid-Open Publication No. 4-122913 discloses a guest host liquid crystal optical element in which two antiferroelectric smectic liquid crystal layers, each including a dichroic dye, are stacked one upon the other such that a normal defined perpendicularly to the smectic layers of one of the liquid crystal layers is parallel to a normal defined perpendicularly to those of the other liquid crystal layer and that the direction to which liquid crystal molecules tilt in response to an electric field applied to one of the two liquid crystal layers is opposite to the direction to which liquid crystal molecules tilt responsive to the electric field applied to the other liquid crystal layer. As opposed to the liquid crystal optical element 500 disclosed in Japanese Laid-Open Publication No. 8-122830, the liquid crystal optical element disclosed in Japanese Laid-Open Publication No. 4-122913 realizes a bright-state display with no voltage applied and a dark-state display with a voltage applied, respectively.
However, the liquid crystal optical element 500 disclosed in Japanese Laid-Open Publication No. 8-122830 uses two liquid crystal cells, thus requiring twice as high a manufacturing cost as the conventional LCD with a single liquid crystal cell.
Japanese Laid-Open Publication No. 4-122913 also discloses an arrangement in which two liquid crystal cells are used to stack two liquid crystal layers one upon the other, thus doubling the manufacturing cost as well as the liquid crystal optical element disclosed in Japanese Laid-Open Publication No. 8-122830.
It should be noted that Japanese Laid-Open Publication No. 4-122913 also discloses an arrangement in which two liquid crystal layers are stacked one upon the other between one pair of substrates. However, a liquid crystal optical element with such a structure cannot be obtained by a normal process step of injecting a liquid crystal material into a predetermined gap between two substrates that have been bonded together. Instead, such a structure may be formed by performing the process step of stacking two filmed liquid crystal layers one upon the other. However, a complicated material processing technique is required to transform an antiferroelectric liquid crystal material into a film shape. Also, to make the filmed liquid crystal layers, a polymer liquid crystal material needs to be used or a network of a polymer material needs to be created in the liquid crystal layers. In that case, the response of the liquid crystal material to the applied voltage will decrease significantly.
Thus, the liquid crystal optical elements that need no polarizers as disclosed in Japanese Laid-Open Publications No. 8-122830 and No. 4-122913 cannot achieve sufficiently high productivity or sufficiently good performance.